1) Field of the Invention
The present invention relates to a charging unit that is used in a charging process, which is one of the processes for forming an image.
2) Description of the Related Art
Electrophotography has been published as one of the processes for forming an image according to the information of an image or a document, in U.S. Pat. No. 2297691, Japanese Patent Publications (Koukoku) 49-23910 and 43-24748.
In electrophotography, generally, an electrostatic latent image is formed by a photo irradiation process according to the information of an image or a document, by exposure or writing on a photoconductive matter of a photoreceptor, which is a latent image carrier. In case of dry developing, the corresponding latent image is developed by using a dry toner, thereby heating, pressurizing in order to fix the processed visual image for copying.
Developing methods are mainly divided into liquid developing and dry developing. The liquid developing method includes cascading that uses a liquid developer composed of an insulating organic liquid, in which pigments, dyes of various types are dispersed minutely. The dry developing method includes magnetic brushing or powder clouding that use toner formed by dispersing colorants like carbon black etc. in a natural or synthetic resin. There are two types of developersxe2x80x94a one-component developer and a two-component developer. The one-component developer contains only toner. The two-component developer contains toner and carrier.
In recent years, there is a tendency towards reducing the particle size of the toner to meet the demand to cope with high image quality. Especially when the latent image is dotted due to digital processing, toner having a small particle size is used, putting an emphasis on acquiring reproducibility and sharpness of dotting.
Japanese Patent Application Laid Open Publication Nos. 1-112253, 2-284158, and 7-295283 propose using the toner having small particle size to achieve a highly defined image having high resolution. These patent publications specify distribution and amount of toner having a small average particle size of less than or equal to 5 xcexcm (micrometer).
The particle size of less than or equal to 5 xcexcm is an essential condition for achieving a highly defined image with high resolution. The toner of this particle size, when supplied for developing of a latent image, proves to be very good in the sense that there is no blurring or distortion of an image and the toner doesn""t go out from the latent image, thus enabling to form an image having superior reproducibility.
Edge effect, which is one of the problems while forming an image, is remarkable when toner of a particle size less than or equal to 5 xcexcm is used. The edge effect can be eliminated by regulating number percentage content of toner particles having a diameter greater than or equal to 5 xcexcm. Concretely, when the average particle size of a particle is less than or equal to 5 xcexcm and number percentage content is between 60% and 80%, a highly defined image with high resolution is achieved. However, when the toner has such a composition, following new problem arises.
It creates a difficulty in setting sufficient charging characteristics required in charging process. The charging process is a process that is carried out after removing toner remained on surface of the photoreceptor after completion of transfer process.
The cleaning, that is the removal of the toner remained on a surface of the photoreceptor, is carried out by wiping the toner off after completion of the transfer process. When the particle size of the toner is too small, the toner is not removed completely by wiping due to improper contact between a surface of the photoreceptor and a blade to be used for wiping. The toner that goes on accumulating on the surface of the photoreceptor easily stains the charging unit, thereby hindering the regular charging of the photoreceptor.
Conventionally, a contact charging method that carries out aerial discharge by micro gap or charge injection by providing a charging member, which is in direct contact or adjacent to the photoreceptor, has been used as one of the charging methods. However, charging is carried out with the charging member almost in direct contact with the photoreceptor, in both charge injection and aerial discharge. Such charging methods are disclosed, for example, in Japanese Patent Application Laid Open Publication No. 63-149668 (Structure with a charging roller), and No. 5-45724 (Structure using a charging brush). Thus, as the toner remained on the surface of the photoreceptor increases, it enters into the area of contact between the charging unit and the photoreceptor, and affects contact of the charging unit with the photoreceptor. This results in a variation in a range of charge injection or uneven discharge, thereby causing a difficulty in maintaining the prescribed charging characteristics.
So far, in order to solve the problems, Japanese Patent Application Laid Open Publication Nos. 7-140762, 7-140868, and 2-301777 have proposed a structure that carries out cleaning of a charging roller provided on the charging unit.
However, according to the structure disclosed in the publications, the cleaning unit that carries out a different function has to be provided on the charging unit, which is meant to carry out the function of charging only. This complicates the structure and also raises the cost.
On the other hand, a decharging process is sometimes carried out along with the wiping of toner in the cleaning process; in order to remove charge remained on a surface of the photoreceptor. If the toner on the photoreceptor can not be removed completely in the cleaning process, the adhesion of the toner to the photoreceptor is weakened due to decharging, and therefore this toner remained, gets scattered from the photoreceptor to the surrounding area due to centrifugal force or air flow inside during the movement of the photoreceptor. This scattered toner or dust may enter into the charging unit, and stick to a charging member thereof.
In some cases, a wire is used as a charging member. This wire is made by drawing a material. If any minute irregularities (micro cracks or scratches caused during processing) in units of microns occurred on the surface of the charging member during the manufacturing process, the toner or dust can easily get into these minute cracks or scratches and stick there. This tendency is significant if a tungsten wire is used as a charging wire. Thus, charging unevenness mentioned above, is attributed to the sticking of toner or dust to the material which is used as a charging member.
When a wire, especially a tungsten wire is used for the charging member, it is possible to lower the discharge voltage by reducing the diameter of the wire. Even if the charge voltage increases in accordance with the increase in particles sticking to the wire during the elapsed time due to a low discharge voltage in the initial recess, partial or sudden arc discharge (leak) is hard to occur, which is an advantage. However, the problem still remains in the strength when the diameter of the wire is reduced. To solve this problem, the charging wire is thickened. However, the discharge voltage is increased, which causes the partial discharge (leak) or the sudden discharge (leak) to easily occur.
A structure for removing the toner or dust forcibly has also been proposed, for example, in Japanese Patent Application Laid Open Publication No. 7-175299 and No. 8-305135, taking into consideration the fact that the sticking of toner or dust resulted due to measurement settings or surface condition during manufacturing process of a wire can not be denied when the wire is used as a charging member. According to these publications, the toner or dust stuck to a surface of the charging wire, is removed by a cleaning device that scrapes the surface of the charging wire. However, there is a possibility of making minute scratches while scraping the surface of the charging wire during cleaning with this structure. Particularly, the volume average particle size between 5 xcexcm and 10 xcexcm is a characteristic of a toner to be used for achieving highly defined image with high resolution. If the toner having 60 to 80 number percentage of the particles having the particle size less than or equal to 5 xcexcm is used, the toner enters into minute scratches that are generated during cleaning. This acts as a core on which scattered toner or floating toner can stick easily to the wire, which may give rise to charging unevenness or arc discharge during the elapsed time.
It is an object of the present invention to solve at least the problems in the conventional technology.
The image forming apparatus according to this invention, performs visible image processing on an electrostatic latent image that is formed on an electrostatic latent image carrier, by using a developer that includes a toner with a volume average particle size between 5 xcexcm and 10 xcexcm and 60 to 80 number percentage particles having a particle size less than or equal to 5 xcexcm. This image forming apparatus includes a charging unit that charges the electrostatic latent image carrier. The charging unit and the electrostatic latent image carrier are not in contact with each other.
The charging device according to another aspect of the present invention uniformly charges an electrostatic latent image carrier to thereby perform visible image processing, on an electrostatic latent image formed on the electrostatic latent image carrier, by using a developer that includes a toner with a volume average particle size between 5 xcexcm and 10 xcexcm and 60 to 80 number percentage particles having a particle size less than or equal to 5 xcexcm. The charging device and the electrostatic latent image carrier are not in contact with each other.
The other objects, features and advantages of the present invention are specifically set forth in or will become apparent from the following detailed descriptions of the invention when read in conjunction with the accompanying drawings.